The Brownells Top Dead Center Punch is an interesting tool. It’s a vee block that has a level visible on the top of the tool, with the vee below. There’s an adjustable stop that comes out from the front of the tool and a punch that rides in a hole perpendicular and centered on the vee. I bought one at a yard sale without knowing what exactly it was, so it took a bit of sleuthing.
The tool is used to mark the location of sight mounting screws on firearms. The receiver is leveled in a vise and the tool allows you to make a punch mark on the centerline of the barrel at top dead center (hence the name). The stop allows you to set the distance of the punchmark from the end of the barrel or other feature.
While certainly useful for this one gunsmithing task, we can think of many applications when working with round parts that need a perpendicular hole. I have vague memories of seeing similar tools for layout on pipes. I had hoped to make this a Dealmonger post but it appears that Brownells is the only supplier of this useful tool.
Top Dead Center Punch [Brownells.com]